Angiogenesis is the process by which new blood vessels are generated. Many of the signaling pathways that drive the angiogenic response originate at the plasma membrane and emanate from the activation of endothelial cell receptor tyrosine kinases, including Tie2/Tek (Jones, N. et al. (2001) Nat. Rev. Mol. Cell. Biol. 4:257-267; Olsson, A. et al. (2006) Nat. Rev. Mol. Cell. Biol. 5:359-371). Angiopoietin (Ang) members 1-4 constitute a family of protein growth factors, all of which have been shown to activate Tie 2 receptor activity to differing extents.
All the Ang's are characterized structurally by an N-terminal super clustering domain (SCD) followed by a coiled-coil domain (CCD) and a C-terminal fibrinogen-like domain (FLD) (Ward, N. and Dumont, D. (2002) Semin. Cell. Dev. Biol. 1:19-27) and (Tsigkos, K. et al. (2003) Expert Opin. Investig. Drugs 6:933-941). Functional studies have highlighted a role for the SCD and CCD's in forming high order homotypic Ang multimers (Procopio, W. et al. (1999) J. Biol. Chem. 42:30196-30201). The specific nature of these multimers is variable and seems to be unique to each Ang family member. Binding specificity of the Ang's for the Tie 2 receptor has been ascribed to the FLD. Taken together, unique structural attributes of each Ang family member promotes binding and differential clustering of Tie 2. The pleiotropic physiological effects of Ang 1-4 are thought to at least in part be mediated by appropriate and specific clustering of the receptor (Davis, S. et. al. (2003) Nat. Struct. Biol. 1:38-44; Procopio, W. et. al. (1999) J. Biol. Chem. 42:30196-30201; Cho, C. et. al. (2004) Proc. Natl. Acad. Sci. USA 15:5547-5552; Ward, D. et. al. (2004) Biochem. Biophys. Res. Commun. 3:937-946; Kim, K-T. et al. (2005) J. Biol. Chem. 280:20126-20131). Gene ablation and transgenic approaches in mice have highlighted an indispensable role for Ang 1 and 2 in the development and maintenance of the blood and lymphatic vascular systems as well as well as the hematopoietic system. Non-genetic studies of the Ang's have been hampered by the inherent difficulty associated with their purification, stability and solubility.
Chronic wounds represent a significant medical problem. For example, global estimates report that 12.5 million patients worldwide suffer from chronic wounds and a significant number of these individuals suffer from decubitus ulcers and diabetic foot ulcers. Wound healing involves a well choreographed series of molecular activities that ultimately lead to wound closure. These events are driven by three interrelated processes: inflammation, cellular proliferation and angiogenesis. Impaired angiogenesis is one of several primary defects reported in diabetic patients. These patients often suffer from impaired wound healing, and as such suffer significant morbidity associated with vascular compromise (Dinh, T. and Veves, A. (2005) Curr. Pharm. Des. 18:145-153).
Primary defects in growth factor secretion and/or proteolytic cleavage of growth factors in diabetic wounds has been reported suggesting therapeutic application of these factors may be beneficial (Wieman, T. et al. (1998) Diabetes Care 5:822-827; Tsang, M. et. al. (2003) Diabetes Care 6:1856-1861). However, to date, effective means for stimulating angiogenesis, such as for use in the treatment of chronic wounds, are still lacking. Accordingly, a need exists for agents that are effective in stimulating angiogenesis.